diff --git a/extension/src/popup/popup.ts b/extension/src/popup/popup.ts
index d098185..c154735 100644
--- a/extension/src/popup/popup.ts
+++ b/extension/src/popup/popup.ts
@@ -67,29 +67,7 @@ function parseUrlParams(): { view?: View; type?: string; id?: string } | null {
 
 // --- State ---
 
-export type View = 'locked' | 'list' | 'detail' | 'add' | 'edit' | 'settings' | 'settings-vault' | 'trash' | 'devices' | 'field-history';
-
-export interface PopupState {
-  view: View;
-  entries: Array<[ItemId, ManifestEntry]>;
-  selectedId: ItemId | null;
-  selectedItem: Item | null;
-  selectedIndex: number;
-  searchQuery: string;
-  activeGroup: string | null;
-  error: string | null;
-  loading: boolean;
-  // Captured tab snapshot taken at popup-open. Used by fill_credentials
-  // to guard against TOCTOU navigation — the SW re-checks this URL's
-  // hostname against the tab's live URL before forwarding fill_credentials
-  // to the content script. See router/popup-only.ts#handleFillCredentials.
-  capturedTabId: number | null;
-  capturedUrl: string;
-  newType: import('../shared/types').ItemType | null;
-  vaultSettings: import('../shared/types').VaultSettings | null;
-  generatorDefaults: import('../shared/types').GeneratorRequest | null;
-  historyItemId: import('../shared/types').ItemId | null;
-}
+import type { View, PopupState } from '../shared/popup-state';
 
 let currentState: PopupState = {
   view: 'locked',
diff --git a/extension/src/shared/__tests__/state.test.ts b/extension/src/shared/__tests__/state.test.ts
new file mode 100644
index 0000000..0a707d2
--- /dev/null
+++ b/extension/src/shared/__tests__/state.test.ts
@@ -0,0 +1,92 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import {
+  registerHost,
+  __resetHostForTests,
+  getState,
+  setState,
+  navigate,
+  sendMessage,
+} from '../state';
+import type { StateHost } from '../state';
+import type { PopupState } from '../popup-state';
+
+function makeHost(initial?: Partial<PopupState>): StateHost {
+  let state: PopupState = {
+    view: 'list',
+    entries: [],
+    selectedId: null,
+    selectedItem: null,
+    selectedIndex: 0,
+    searchQuery: '',
+    activeGroup: null,
+    error: null,
+    loading: false,
+    capturedTabId: null,
+    capturedUrl: '',
+    newType: null,
+    vaultSettings: null,
+    generatorDefaults: null,
+    historyItemId: null,
+    ...initial,
+  };
+
+  return {
+    getState: () => state,
+    setState: (partial) => { state = { ...state, ...partial }; },
+    navigate: vi.fn(),
+    sendMessage: vi.fn().mockResolvedValue({ ok: true }),
+    escapeHtml: (s) => s,
+    popOutToTab: vi.fn(),
+    isInTab: () => false,
+    openVaultTab: vi.fn(),
+  };
+}
+
+describe('shared/state', () => {
+  beforeEach(() => {
+    __resetHostForTests();
+  });
+
+  it('register-then-getState round-trips', () => {
+    const host = makeHost({ view: 'detail' });
+    registerHost(host);
+    expect(getState().view).toBe('detail');
+  });
+
+  it('double-register throws', () => {
+    registerHost(makeHost());
+    expect(() => registerHost(makeHost())).toThrow(/already registered/);
+  });
+
+  it('__resetHostForTests clears the singleton', () => {
+    registerHost(makeHost());
+    __resetHostForTests();
+    expect(() => getState()).toThrow(/No state host/);
+  });
+
+  it('getState without host throws', () => {
+    expect(() => getState()).toThrow(/No state host/);
+  });
+
+  it('setState merges partial state', () => {
+    const host = makeHost();
+    registerHost(host);
+    setState({ loading: true });
+    expect(getState().loading).toBe(true);
+  });
+
+  it('navigate delegates to host', () => {
+    const host = makeHost();
+    registerHost(host);
+    navigate('settings');
+    expect(host.navigate).toHaveBeenCalledWith('settings', undefined);
+  });
+
+  it('sendMessage delegates to host', async () => {
+    const host = makeHost();
+    registerHost(host);
+    const resp = await sendMessage({ type: 'is_unlocked' });
+    expect(host.sendMessage).toHaveBeenCalledWith({ type: 'is_unlocked' });
+    expect(resp).toEqual({ ok: true });
+  });
+});
diff --git a/extension/src/shared/popup-state.ts b/extension/src/shared/popup-state.ts
new file mode 100644
index 0000000..09e94e7
--- /dev/null
+++ b/extension/src/shared/popup-state.ts
@@ -0,0 +1,57 @@
+// State shared between popup and vault surfaces. Kept here (not in popup/) so
+// shared/state.ts can import without creating a popup→shared circular dep.
+
+import type {
+  Item,
+  ItemId,
+  ItemType,
+  ManifestEntry,
+  GeneratorRequest,
+  VaultSettings,
+} from './types';
+
+export type View =
+  | 'locked'
+  | 'list'
+  | 'detail'
+  | 'add'
+  | 'edit'
+  | 'settings'
+  | 'settings-vault'
+  | 'trash'
+  | 'devices'
+  | 'field-history'
+  // Vault-tab-only views; popup never navigates to these. Kept in the union so
+  // a single typed StateHost contract serves both surfaces (popup + vault).
+  | 'history'
+  | 'backup'
+  | 'import';
+
+export interface PopupState {
+  view: View;
+  entries: Array<[ItemId, ManifestEntry]>;
+  selectedId: ItemId | null;
+  selectedItem: Item | null;
+  selectedIndex: number;
+  searchQuery: string;
+  activeGroup: string | null;
+  error: string | null;
+  loading: boolean;
+  // Captured tab snapshot taken at popup-open. Used by fill_credentials
+  // to guard against TOCTOU navigation — the SW re-checks this URL's
+  // hostname against the tab's live URL before forwarding fill_credentials
+  // to the content script. See router/popup-only.ts#handleFillCredentials.
+  capturedTabId: number | null;
+  capturedUrl: string;
+  newType: ItemType | null;
+  vaultSettings: VaultSettings | null;
+  generatorDefaults: GeneratorRequest | null;
+  historyItemId: ItemId | null;
+  // Vault-tab-only fields. The popup surface leaves these at their defaults
+  // (unlocked=false implicit via separate lock-screen view, drawer/panel false).
+  // Kept on the shared shape so VaultState satisfies StateHost.getState()
+  // without a cast.
+  unlocked?: boolean;
+  drawerOpen?: boolean;
+  typePanelOpen?: boolean;
+}
diff --git a/extension/src/shared/state.ts b/extension/src/shared/state.ts
index 07f914b..30deeaf 100644
--- a/extension/src/shared/state.ts
+++ b/extension/src/shared/state.ts
@@ -1,15 +1,21 @@
-/// Service-locator for cross-bundle state access.
-///
-/// Both popup.ts and vault.ts register themselves as the "host".
-/// All popup components import from here instead of from popup.ts,
-/// so the same component code works in either bundle.
+// extension/src/shared/state.ts
+//
+// Single channel for popup and vault-tab UI to read/write app state and
+// dispatch messages to the service worker. Two registered hosts (popup,
+// vault tab) implement StateHost; each surface calls registerHost(this) at
+// boot.
+//
+// The vault_locked intercept (lines 47-74 in vault.ts pre-Phase-4) lifts
+// into sendMessage() here in Phase 4. Phase 1 lays the wrapper signature;
+// the body is a thin pass-through until Phase 4.
 
 import type { Request, Response } from './messages';
+import type { PopupState, View } from './popup-state';
 
 export interface StateHost {
-  getState(): any;
-  setState(partial: any): void;
-  navigate(view: string, extras?: any): void;
+  getState(): PopupState;
+  setState(partial: Partial<PopupState>): void;
+  navigate(view: View, extras?: Partial<PopupState>): void;
   sendMessage(request: Request): Promise<Response>;
   escapeHtml(s: string): string;
   popOutToTab(): void;
@@ -19,24 +25,36 @@ export interface StateHost {
 
 let host: StateHost | null = null;
 
-export function registerHost(h: StateHost): void { host = h; }
+export function registerHost(h: StateHost): void {
+  if (host) throw new Error('state host already registered');
+  host = h;
+}
 
-export function getState(): any {
+/** Test-only — vitest beforeEach() calls this to break inter-test leakage. */
+export function __resetHostForTests(): void {
+  host = null;
+}
+
+export function getState(): PopupState {
   if (!host) throw new Error('No state host registered');
   return host.getState();
 }
 
-export function setState(partial: any): void {
+export function setState(partial: Partial<PopupState>): void {
   if (!host) throw new Error('No state host registered');
   host.setState(partial);
 }
 
-export function navigate(view: string, extras?: any): void {
+export function navigate(view: View, extras?: Partial<PopupState>): void {
   if (!host) throw new Error('No state host registered');
   host.navigate(view, extras);
 }
 
-export function sendMessage(request: Request): Promise<Response> {
+/**
+ * Phase 4 will add a vault_locked intercept here. For now, this is a pure
+ * pass-through so the signature is stable for Phase 4 to fill.
+ */
+export async function sendMessage(request: Request): Promise<Response> {
   if (!host) throw new Error('No state host registered');
   return host.sendMessage(request);
 }
@@ -52,7 +70,7 @@ export function popOutToTab(): void {
 }
 
 export function isInTab(): boolean {
-  if (!host) return false;
+  if (!host) throw new Error('No state host registered');
   return host.isInTab();
 }
 
diff --git a/extension/src/vault/vault.ts b/extension/src/vault/vault.ts
index 9bbdc57..421c918 100644
--- a/extension/src/vault/vault.ts
+++ b/extension/src/vault/vault.ts
@@ -229,11 +229,11 @@ const state: VaultState = {
 
 registerHost({
   getState: () => state,
-  setState: (partial: any) => {
+  setState: (partial) => {
     Object.assign(state, partial);
     renderPane();
   },
-  navigate: (view: string, extras?: any) => {
+  navigate: (view, extras) => {
     Object.assign(state, { view, error: null, loading: false, ...extras });
     setHash(view as VaultView);
     applyShellViewClass();